| Stefan Zager | ce1b867 | 2023-03-13 17:08:07 | [diff] [blame] | 1 | // Copyright 2023 The Chromium Authors |
| 2 | // Use of this source code is governed by a BSD-style license that can be |
| 3 | // found in the LICENSE file. |
| 4 | |
| 5 | #ifndef BASE_MEMORY_STACK_ALLOCATED_H_ |
| 6 | #define BASE_MEMORY_STACK_ALLOCATED_H_ |
| 7 | |
| Avi Drissman | 5e5f3fb0 | 2023-09-14 14:47:07 | [diff] [blame] | 8 | #include <stddef.h> |
| 9 | |
| Stefan Zager | ce1b867 | 2023-03-13 17:08:07 | [diff] [blame] | 10 | #if defined(__clang__) |
| 11 | #define STACK_ALLOCATED_IGNORE(reason) \ |
| 12 | __attribute__((annotate("stack_allocated_ignore"))) |
| 13 | #else // !defined(__clang__) |
| 14 | #define STACK_ALLOCATED_IGNORE(reason) |
| 15 | #endif // !defined(__clang__) |
| 16 | |
| 17 | // If a class or one of its ancestor classes is annotated with STACK_ALLOCATED() |
| 18 | // in its class definition, then instances of the class may not be allocated on |
| 19 | // the heap or as a member variable of a non-stack-allocated class. |
| 20 | #define STACK_ALLOCATED() \ |
| 21 | public: \ |
| 22 | using IsStackAllocatedTypeMarker [[maybe_unused]] = int; \ |
| 23 | \ |
| 24 | private: \ |
| 25 | void* operator new(size_t) = delete; \ |
| 26 | void* operator new(size_t, ::base::NotNullTag, void*) = delete; \ |
| 27 | void* operator new(size_t, void*) = delete |
| 28 | |
| 29 | namespace base { |
| 30 | |
| 31 | // NotNullTag was originally added to WebKit here: |
| 32 | // https://trac.webkit.org/changeset/103243/webkit |
| 33 | // ...with the stated goal of improving the performance of the placement new |
| 34 | // operator and potentially enabling the -fomit-frame-pointer compiler flag. |
| 35 | // |
| 36 | // TODO(szager): The placement new operator which uses this tag is currently |
| 37 | // defined in third_party/blink/renderer/platform/wtf/allocator/allocator.h, |
| 38 | // in the global namespace. It should probably move to /base. |
| 39 | // |
| 40 | // It's unknown at the time of writing whether it still provides any benefit |
| 41 | // (or if it ever did). It is used by placing the kNotNull tag before the |
| 42 | // address of the object when calling placement new. |
| 43 | // |
| 44 | // If the kNotNull tag is specified to placement new for a null pointer, |
| 45 | // Undefined Behaviour can result. |
| 46 | // |
| 47 | // Example: |
| 48 | // |
| 49 | // union { int i; } u; |
| 50 | // |
| 51 | // // Typically placement new looks like this. |
| 52 | // new (&u.i) int(3); |
| 53 | // // But we can promise `&u.i` is not null like this. |
| 54 | // new (base::NotNullTag::kNotNull, &u.i) int(3); |
| 55 | enum class NotNullTag { kNotNull }; |
| 56 | |
| 57 | } // namespace base |
| 58 | |
| 59 | #endif // BASE_MEMORY_STACK_ALLOCATED_H_ |